Evolutionary change by means of Natural Selection needs a couple of things in order to happen: heritability and variation in fitness. That is, offspring need to resemble their parents at least a little (heritability) and individuals need to differ in their survival and offspring production (fitness). We’ll worry about heritability in another post, but variation is something that seems like it might be hard to maintain. Some forms of Natural Selection will reduce variation as more fit individuals become frequent and all the different kinds of less fit individuals are eliminated from the population. However, there is a force, common in nature, which may maintain variation, parasites.
Interactions between hosts and parasites can generate strong selective pressures on each player, especially if your life depends on infecting a host. Often, biologists make an analogy to an arms race where players are developing bigger and better defenses or weapons. Antagonistic interactions may also generate negative frequency dependence where a rare host type is favored because the parasites are adapted to a common type. You can learn more by checking out CJ’s post on the Red Queen Hypothesis or Jeremy’s post on a different coevolutionary puzzle. A key component for maintaining variation via negative frequency dependent selection is specificity. There must variation in the interaction among different host genotypes and parasite genotypes. This is sometimes referred to as a GxG interaction. If parasites can infect all the hosts, there is no specificity. Specificity allows different hosts to be favored over time depending on the composition of the parasite population.
Theoreticians love to use different models of interactions between hosts and parasites, but without empirical evidence, there seems little point. In a recent paper by Rouchet and Vorburger (2012), the authors looked for evidence of just the kind of genetic specificity would result in the maintenance of genetic variation.
What happens when two parasites infect the same host individual? Is the outcome similar to the Thunderdome: two parasites enter, one parasite leaves? Host-parasite interactions are rarely so simple. While a reductionist approach to understanding the interaction of a parasite or pathogen with its host may decompose the system to a single infection, nature is full of much more complex puzzles. Within the host, the battle itself raging between parasites (within-host competition) may have cascading effects on the host.
A recent paper on virulence caught my eye (Bashey et al., 2012) which provides an update to a very interesting result from the group a few years ago. The system includes bacterial parasites, along with parasitic nematodes, that infect insect larvae and eat/digest them from the inside out. Vigneux et al. (2008) found that when multiple parasite isolates are mixed in a host, the host mortality decreased. However, this only occurred when the isolates were not related. In the experiment, the researchers created low relatedness by mixing populations with migration. I reviewed the 2008 paper over at the Coevolvers blog, my personal science blog. The hypothesis was that chemical warfare among the parasites decreased the parasite load and reduced the negative effects on the host, virulence.
What are the evolutionary consequences of parasite superinfection (i.e. simultaneous infection by multiple parasites)? When parasites are genetically distinct, coexistence within a host generates conflict because of limited resources. How this conflict is resolved is the source of evolutionary research on the evolution of parasite life history traits such as virulence, the negative effects on the host caused by infection, and transmission mode, how parasites infect a new host. The transmission mode of a parasite is often characterized as occurring in one of two different modes: vertical or horizontal. With vertical transmission, an offspring obtains its parasites directly from its parents. In contrast, with horizontal transmission, infections occur either directly from the environment or contagiously by infection from other individuals.
My interest in the evolution of transmission mode in parasites and symbionts led me to a recent paper (Ben-Ami et al. 2011), which addresses the consequences of superinfection by two different parasites with different transmission modes of the waterflea, Daphnia magna, on virulence and parasite fecundity. Pasteuria ramosa is a castrating, horizontally transmitted, blood-infecting bacterium where spores are produced from the cadaver of the host Daphnia. Octosporea bayeri, a microsporidium, utilizes both vertical transmission to eggs and horizontal transmission via waterborne spores.